This lichen is called Map Lichen (Rhizocarpon Geographicum), a lichen that grows on quartz. Lichens are sensitive to air pollution and their presence or absence is used as an indicator. For example some types of lichen fade away in areas where the concentration of sulphur dioxide (SO2) is high. Beard mosses and bush-forming lichens in particular turn out to be very sensitive to air pollution, crustaceous lichens seem less susceptible. In the 1960s and 1970s, due to air pollution, the number of species of epiphytic lichen had diminished so much in large parts of the Netherlands, that these areas were referred to as 'epiphyte deserts'.